The present disclosure relates, in general, to photonic devices, and more particularly to improved photonic devices and methods for fabricating them.
Semiconductor lasers typically are fabricated on a wafer by growing an appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an epitaxial structure having an active layer parallel to the substrate surface. The wafer is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer and incorporating metallic contacts attached to the semiconductor material. Laser facets are typically formed at the ends of the laser cavity by cleaving the semiconductor material along its crystalline structure to define edges, or ends, of the laser optical cavity so that when a bias voltage is applied across the contacts, the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow. Since the semiconductor material is cleaved to form the laser facets, the locations and orientations of the facets are limited; furthermore, once the wafer has been cleaved, typically it is in small pieces so that conventional lithographical techniques cannot readily be used to further process the lasers.
The foregoing and other difficulties resulting from the use of cleaved facets led to the development of a process for forming the facets of semiconductor lasers through etching. This process, as described in U.S. Pat. No. 4,851,368, also allows lasers to be monolithically integrated with other photonic devices on the same substrate, the disclosure of which is hereby incorporated herein by reference. This work was further extended and a ridge laser process based on etched facets was disclosed in the IEEE Journal of Quantum Electronics, volume 28, No. 5, pages 1227-1231, May 1992.
One of the major challenges in the use of semiconductor lasers is the mismatch between the output beam from the laser and the medium to which the beam is directed or coupled. For example, forming a semiconductor laser with spot size converters (SSC) can allow more efficient coupling of the laser light to an optical fiber or expand the tolerance for optical alignment, however, in general there are certain disadvantages that come along with forming SSC, such as process complexity and degradation in laser characteristics. An example of the degradation in laser characteristics is the increase in the laser threshold current. The following publications discuss the various SSC approaches employed: “Spot-Size Converter Integrated Laser Diodes (SS-LD's)” by Itaya, et al., IEEE Journal of Selected Topics in Quantum Electronics, Volume 3, Number 3, pages 968-974; “A Review on Fabrication Technologies for the Monolithic Integration of Tapers with III-V Semiconductor Devices” by Moerman, et al., IEEE Journal of Selected Topics in Quantum Electronics, Volume 3, Number 6, pages 1308-1320; and “1.3-μm Spot-Size-Converter Integrated Laser Diodes Fabricated by Narrow-Stripe Selective MOVPE” by Yamazaki, et al., IEEE Journal of Selected Topics in Quantum Electronics, Volume 3, Number 6, pages 1392-1398.
A laser structure formed through a simple process that allows beam modification without significant impact to laser characteristics, such as laser threshold, is very desirable, and, for example, can lead to very efficient coupling of the laser beam into an optical fiber with low cost packaging.